How to obtain clues from the otoliths of an adult fish about the aquatic environment it has been in as a larva

Analyses by mass spectrometry of carbon/oxygen isotope compositions of lapilli and sagittal otoliths in the catadromous New Zealand mullet Mugil cephalus permit us to draw the following conclusions:

• 1.1. Information on the type of aquatic environment a larval fish has experienced can be obtained from otoliths of the adult in two ways: drilled cores along the long axis of the sagitta can be analysed sequentially for carbon/oxygen isotope composition.
• 2.2. The oldest isotope ratios, representing the time when the first aragonite was laid down, are closest to the centre, while material nearer to the edges of the earstone reflects carbon/oxygen isotope values of the environment the fish had experienced at the time of capture.
• 3.3. The second approach involves the lapilli. These otoliths are universally smaller than the sagittae in adult fish, but at hatching the two are of equal size in many species of fish, including the mullet.
• 4.4. Consequently, the lapillus carbon/oxygen isotope ratio closely resembles that of the larval environment (in our case, the estuarine waters of New Zealand's North Island), whereas averaged carbon/oxygen isotope ratios of the sagittae more closely agree with those of river water.